1. Field of the Invention
The present invention relates to an optical disk apparatus equipped with an optical pickup in which an objective is displaced with respect to an incident laser beam during servo tracking and the detection of tracking error signals is effected according to the so-called push-pull system.
2. Description of the Prior Art
It has been known that there are two problems associated with this kind of optical disk apparatus as will be explained hereunder.
One problem is that if the push-pull system which is the simplest in structure and achieves an accurate result is adopted as a tracking error detecting means, tracking error signals detected include direct-current offset and therefore a correct servo tracking driving of the objective cannot be achieved.
The detection of tracking errors using the push-pull system may be effected, as seen from FIG. 43, in a manner in which a beam spot is formed on an optical disk D by means of an objective 1, then reflected towards a beam splitter 2 disposed between a laser emitting device (not shown in FIG. 43) and the objective 1 and the beam splitter 2 transmits the beam to a binary divided photodetector 3. A pit and/or pregrooves formed on the optical disk D causes diffraction of the laser beam and for that reason, the diffraction image formed on the binary divided photodetector 3 has a dark portion and bright portion depending on the relative positions of the spot S and pit P on the optical disk D. Some examples of such relative positions of the spot and the pit are illustrated in FIGS. 44(a), 45(a) and 46(a), while each corresponding diffraction image S' is schematically shown in FIG. 44(b), FIG. 45(b) or FIG. 46(b). In each figure (b) of FIGS. 44 to 46, the dark portion is depicted as a shaded area and if the position of a spot S coincides with that of a pit P as shown in FIG. 45(a), a dark image such as that shown in FIG. 45(b) is formed on the photodetector 3.
As will be seen from FIGS. 47 and 48, the binary divided photodetector 3 has two light receiving portions 3a and 3b arranged in a parallel relation along the direction corresponding to the longitudinal direction of the image of the pit P projected on the diffraction image S', each of which outputs electric signals in proportion to the intensity of light received.
If the position of a spot S coincides with that of a pit P, the electric signal of one portion 3a is equal to that of other portion 3b. And if the position of a spot S drifts from that of a pit P in a right or left direction, the electric signal of one portion can be increased, and that of the other portion can be decreased.
These electric signals are processed with a subtracter to determine the relative positions of the spot S and the pit P and the subtracter outputs the detected relative positions as the tracking error signals.
If a tracking error is detected and only the objective 1 in the optical systems is moved to a position, shown by broken line in FIG. 43 so as to compensate for the detected tracking error, the reflected beam changes the light path (see broken line in FIG. 43) while the positions of optical systems other than the objective 1 remain unchanged and therefore, the position of diffraction images S' formed on the photodetector 3 is, as a whole, displaced to that shown by the broken line in FIG. 48. On the other hand, the correct position of the diffraction image which is observed if the optical axis O.sub.1 of the objective 1 coincides with the center line of the laser beam is shown in FIG. 47 (see alternate long and shot dash line.)
Moreover, a luminous flux transmitted to the objective 1 has a intensity distribution as is shown in FIG. 43 with a peak at the center. Therefore, when the objective 1 is moved, for example, by tracking on the radial runout pit and/or pregroove, the intensity distribution of the luminous flux transmitted to the objective 1 causes a change and in turn there is observed a change of the luminance in the diffraction image S' formed on the binary divided photodetector 3 for reading signals.
Consequently, the tracking error signals outputted from the subtracter become non-uniform because of the presence of direct-current offset as is shown in FIG. 41, even if the positions of the spot S and the pit P are aligned as shown in FIG. 45(a). Thus, the position of spot S cannot correctly be aligned with that of pit P even when the objective 1 is displaced on the basis of such tracking error signal.
The second problem associated with the conventional optical disk apparatus is observed when the optical pickup is moved at an extremely high speed or is abruptly stopped during accessing the spot formed by the objective to a desired track. That is, under such condition, the portion supporting the objective causes vibration and then the stationary state is achieved according to an attenuation vibration curve shown in FIG. 42 (see solid curve). This means that a relatively long time interval is required until the position of beam irradiating the optical disk D is completely stabilized and this is turn means that the reading of signals from the optical disk D cannot be effected immediately after access.